First Law of Thermodynamics: Energy Conservation For Mechanical and Industrial Engineering
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The document provides information on the first law of thermodynamics and energy conservation. It defines different forms of energy including kinetic energy, potential energy, and internal energy. It discusses how energy can be transferred between systems through work and heat. Several examples are provided to illustrate calculating changes in kinetic and potential energy, work, power, heat transfer, and applying the first law of thermodynamics to closed systems undergoing thermodynamic processes and cycles.
First Law of Thermodynamics: Energy Conservation For Mechanical and Industrial Engineering
- 1. Lesson 2: First Law of Thermodynamics: Energy Conservation 1 Taught By Meng Chamnan,
- 2. Energy; what is it? 2By Meng Chamnan GIM 2 2 2 1 2 1 1 2 KE KE KE m V V kinetic energy, KE, of the body. The change in kinetic energy, ΔKE gravitational potential energy, PE. The change in gravitational potential energy, ΔPE 2 1 2 1PE PE PE mg z z A basic idea is that energy can be stored within systems in various forms. Energy also can be converted from one form to another and transferred between systems. For closed systems, energy can be transferred by work and heat transfer.
- 3. 3 Example 1: a. To illustrate the proper use of units in the calculation of such terms, consider a system having a mass of 1 kg whose velocity increases from 15 m/s to 30 m/s while its elevation decreases by 10 m at a location where g = 9.7 m/s2. b. For a system having a mass of 1 lb whose velocity increases from 50 ft/s to 100 ft/s while its elevation decreases by 40 ft at a location where g = 32.0 ft/s2. Both a and b, what are the value of ΔKE and ΔPE? By Meng Chamnan GIM
- 4. Work work done by the force can be considered a transfer of energy to the body, where it is stored as gravitational potential energy and/or kinetic energy. 2 1 dsFW 4By Meng Chamnan GIM W > 0: work done by the system W < 0: work done on the system
- 5. Power By Meng Chamnan GIM 5 The rate of energy transfer by work is called power and is denoted by when a work interaction involves an observable force; W F V Power Transmitted by a Shaft T = FtR tW F V R R T T Electric Power. W i
- 6. 6 Example 2: Let us evaluate the power required for a bicyclist traveling at 5.6 m/s to overcome the drag force imposed by the surrounding air. This aerodynamic drag force is given by , where CD is a constant called the drag coefficient, A is the frontal area of the bicycle and rider, and ρ is the air density. Calculate the power that required for a bicyclist if using typical values: CD = 0.88, A = 1.2 m2, and ρ = 0.0005 kg/m3 together with V = 5.6 m/s. By Meng Chamnan GIM 21 2 D DF C A V
- 7. 7 Example 3: A 12-V automotive storage battery is charged with a constant current of 2 amp for 24 h. If electricity costs $0.08 per kW·h, determine the cost of recharging the battery. By Meng Chamnan GIM
- 8. Work: Expansion or Compression 8By Meng Chamnan GIM W pAdx W pdV 2 1 V V W pdV
- 9. 9 Example 4: A gas in a piston–cylinder assembly undergoes an expansion process for which the relationship between pressure and volume is given by The initial pressure is 3 bar, the initial volume is 0.1 m3, and the final volume is 0.2 m3. Determine the work for the process, in kJ, if (a) n = 1.5, (b) n = 1.0, and (c) n = 0. By Meng Chamnan GIM
- 10. Energy Transfer by Heat By Meng Chamnan GIM 10 Q > 0: heat transfer to the system Q < 0: heat transfer from the system adiabatic process there is no energy transfer by heat
- 11. Form of Energy Energy Kinetics Potential Internal Bulk motion Position (level) All other than kinetics and potential 11By Meng Chamnan GIM
- 12. Energy Accounting: Energy Balance for Closed Systems By Meng Chamnan GIM 12 KE PE U Q W
- 13. 13 Example 5: Four kilograms of a certain gas is contained within a piston– cylinder assembly. The gas undergoes a process for which the pressure–volume relationship is The initial pressure is 3 bar, the initial volume is 0.1 m3, and the final volume is 0.2 m3. The change in specific internal energy of the gas in the process is u2 - u1 = - 4.6 kJ/kg. There are no significant changes in kinetic or potential energy. Determine the net heat transfer for the process, in kJ. By Meng Chamnan GIM
- 14. 14 Example 6: Air is contained in a vertical piston–cylinder assembly fitted with an electrical resistor. The atmosphere exerts a pressure of 14.7 lbf/in.2 on the top of the piston, which has a mass of 100 lb and a face area of 1 ft2. Electric current passes through the resistor, and the volume of the air slowly increases by 1.6 ft3 while its pressure remains constant. The mass of the air is 0.6 lb, and its specific internal energy increases by 18 Btu/lb. The air and piston are at rest initially and finally. The piston–cylinder material is a ceramic composite and thus a good insulator. Friction between the piston and cylinder wall can be ignored, and the local acceleration of gravity is g = 32.0 ft/s2. By Meng Chamnan GIM
- 15. 15 Example 6 (continue…) Determine the heat transfer from the resistor to the air, in Btu, for a system consisting of (a) the air alone, (b) the air and the piston. By Meng Chamnan GIM
- 16. 16 Example 7: During steady-state operation, a gearbox receives 60 kW through the input shaft and delivers power through the output shaft. For the gearbox as the system, the rate of energy transfer by heat is , where h is a constant, is the outer surface area of the gearbox, Tb = 300 K (27°C) is the temperature at the outer surface, and Tf = 293 K (20°C) is the temperature of the surrounding air away from the immediate vicinity of the gearbox. For the gearbox, evaluate the heat transfer rate and the power delivered through the output shaft, each in kW. b fQ hA T T By Meng Chamnan GIM
- 17. 17 Example 8: A silicon chip measuring 5 mm on a side and 1 mm in thickness is embedded in a ceramic substrate. At steady state, the chip has an electrical power input of 0.225 W. The top surface of the chip is exposed to a coolant whose temperature is 20°C. The rate of energy transfer by heat between the chip and the coolant is given by , where Tb and Tf are the surface and coolant temperatures, respectively, A is the surface area, and If heat transfer between the chip and the substrate is negligible, determine the surface temperature of the chip, in °C. b fQ hA T T By Meng Chamnan GIM
- 18. Energy Analysis of Cycles By Meng Chamnan GIM 18
- 19. 19By Meng Chamnan GIM Systems undergoing cycles of the type deliver a net work transfer of energy to their surroundings during each cycle cycle in outW Q Q Thermal efficiency: Power Cycles 1 cycle in out out in in in W Q Q Q Q Q Q
- 20. Refrigeration and Heat Pump Cycles By Meng Chamnan GIM 20 cycle out inW Q Q COP of Refrigeration Cycles COP of Heat Pump Cycles in in cycle out in Q Q W Q Q out out cycle out in Q Q W Q Q
- 21. 21 Example 9: Each line in the following table gives information about a process of a closed system. Every entry has the same energy units. Fill in the blank spaces in the table. By Meng Chamnan GIM
- 22. 22 Example 10: By Meng Chamnan GIM Wcycle ? η ? Qin ? Wcycle ?
- 23. 23 Example 11: By Meng Chamnan GIM Qin ? Wcycle ? Qin ? β ?
- 24. 24 Example 12: A gas undergoes a thermodynamic cycle consisting of three processes: Process 1–2: constant volume, V = 0.028 m3, U2 - U1 = 26.4 kJ Process 2–3: expansion with pV = constant, U3 = U2 Process 3–1: constant pressure, p = 1.4 bar, W31 = -10.5 kJ There are no significant changes in kinetic or potential energy. (a) Sketch the cycle on a p–V diagram. (b) Calculate the net work for the cycle, in kJ. (c) Calculate the heat transfer for process 2–3, in kJ. (d) Calculate the heat transfer for process 3–1, in kJ. Is this a power cycle or a refrigeration cycle? By Meng Chamnan GIM